Barack Obama wants a world without nuclear weapons. America will push the idea of "global zero" at the United Nations conference on nuclear non-proliferation that opened in New York yesterday. The vision was unveiled just over a year ago. In a speech in Prague, the US president painted a glorious picture of a world freed from the nuclear threat, while adding (in words that faintly echoed Martin Luther King) that it might not happen in his lifetime.

It was good stuff. But I sincerely hope that Mr Obama was being insincere. For the idea of a world free of nuclear weapons is not so much an impossible dream as an impossible nightmare.

Mr Obama's distant vision matters because nuclear weapons are once again at the very centre of global politics. Yesterday the propaganda war raged in New York, as first Mahmoud Ahmadi-Nejad, the president of Iran, and then Hillary Clinton, the US secretary of state, took to the podium at the UN.